This invention relates to a process and composition for cladding optical fibers.
As discussed in, for example, Blyler and Aloisio, Polymer coatings for optical fibers, Chemtech, November 1987, pages 680-684, optical fibers consist of a central core, usually a highly transparent glass (silica, often containing various doping materials) surrounded by a cladding with a refractive index lower than that of the glass; this cladding serves to confine light within the central core in order to reduce radiation losses from the surface of the core, and hence reduce attenuation of radiation travelling along the core.
In most optical fibers, the cladding is formed from a second glass. Because minor flaws in such a glass cladding greatly reduce the tensile strength of the optical fiber, it is customary to provide the fiber with a secondary, protective cladding, which is usually formed from a polymeric material. For example, U.S. Pat. No. 4,558,082 and U.S. Pat. No. 4,663,185 describe acrylated silicone polymers useful as, inter alia, optical fiber claddings. These silicone polymers are prepared by reacting limonene oxide-functional silicones with acrylic acid or a substituted acrylic acid in the presence of a catalyst, which can be a tetraalkylurea or a tetraalkylguanidine.
Acrylic resins have also been used as protective claddings for optical fibers. U.S. Pat. No. 4,427,823 describes an uncured, filled coating composition comprising 100 parts by weight of (a) a polyfunctional acrylic-type carboxylic acid ester monomer or its prepolymer, this monomer or prepolymer being composed of 0 to 75 percent by weight tri- or higher acrylates, and 25 to 100 percent diacrylate; (b) 0.001 to 20 parts by weight of a polymerization initiator; and (c) 5 to 250 parts by weight of an inorganic solid filler having a refractive index of 1.40 to 1.60 and an average first-order particle diameter of at least 1 m.mu. but less than 1.mu..
U.S. Pat. No. 4,479,984 describes multifilament bundles (which can be optical fiber bundles) impregnated with an ultraviolet curable resin to form a composite material suitable for use as a strength member. Among the resins which can be used in such bundles are various acrylate resins.
U.S. Pat. No. 4,690,503 describes a glass optical fiber having a primary coating constructed of two layers of ultraviolet cured acrylate resin. The first, inner layer has a modulus of elasticity at 25.degree. C. less than or equal to 5 N/mm.sup.2, while the second, outer layer has a modulus of elasticity at 25.degree. C. of from 25 to 1500 N/mm.sup.2, the ratio of the thickness of the first layer to the thickness of the second layer being between 0.5 and 2.
In some cases it is possible to form the primary cladding of the optical fiber (i.e., the cladding immediately adjacent the core) from a polymeric material. According to the aforementioned Blyler and Aloisio article, polymer-clad fibers usually consist of a silica core clad with either a poly(dimethylsiloxane) resin or a fluorinated acrylic polymer. For example, U.S. Pat. No. 4,568,566 describes photocurable silicone compositions, useful as optical fiber claddings, which compositions contain chemically combined siloxy units and units of the formula R.sub.2 SiO, where a number of the R units are acrylate or alkyl-substituted acrylate ester groupings.
U.S. Pat. No. 4,554,339 and U.S. Pat. No. 4,597,987 describe organopolysiloxanes having a viscosity of 100 mPa at 25.degree. C. and having both SiC-bonded acryloxyalkyl groups and Si-bonded hydrogen atoms in the same molecule. These organopolysiloxanes are prepared by adding an allyl alcohol to a diorganopolysiloxane containing a terminal Si-bonded hydrogen atom, then esterifying the hydroxyl groups of the resultant reaction product with acrylic acid and subsequently equilibriating the resultant diorganopolysiloxane with an organo(poly) siloxane containing an Si-bonded hydroxyl group in each of the terminal units. The final organopolysiloxane is stated to be useful as, inter alia, an optical fiber cladding.
However, silicone primary claddings have a number of serious disadvantages. The viscosity and curing requirements of the silicones restrict the production rate of the clad fiber to about 0.5 meters/sec. Silicone claddings do not adhere well to quartz, and the softness of the cladding leads to difficulties in connecting the clad fiber to other components of the optical system; temperature changes can cause the quartz core to be forced into and out of the cladding at the connection. Furthermore, according to U.S. Pat. No. 4,511,209, exposing the silicone-clad optical fibers to low temperatures in the range of -40.degree. to -50.degree. C. results in an increase in attenuation of 10-20 dB/km; in many cases an increase in room temperature attenuation occurs after the fiber has been exposed to such low temperatures.
It is also known that fluorine-containing materials can be incorporated into claddings containing acrylates and methacrylates. For example, U.S. Pat. No. 4,508,916 describes curable substituted urethane acrylates and methacrylates having an aliphatic backbone with at least one pendant fluorinated organic group attached thereto, this backbone being end-capped with an acrylic or methacrylic group.
U.S. Pat. No. 4,617,350 describes a thermoplastic resin useful for optical purposes, including optical fiber claddings, and obtained by blending a polymer of an acrylic ester with a copolymer of vinylidene fluoride and hexafluoroacetone. The refractive index of the blend is in the range of 1.37 to 1.48.
U.S. Pat. No. 4,914,171 describes a difunctional epoxy acrylate monomer prepared by reacting a highly fluorinated diglycidyl ether with an excess of acrylic acid in the presence of a catalyst. The patent states that clear, colorless, low refractive index polymers can be prepared by polymerization of this monomer or blends with other acrylates to give products useful as low surface energy coatings, and low refractive index coatings.
Copending application Ser. No. 07/521,671, filed May 10, 1990 and assigned to the same assignee as the present application (now U.S. Pat. No. 5,024,507) describes and claims a photopolymerizable composition capable of being polymerized upon exposure to ultraviolet light, the composition forming upon photocuring a cured composition having a refractive index not greater than about 1.43, and comprising a substantially homogeneous mixture of:
a) an unsubstituted or fluorosubstituted diacrylate monomer; PA1 b) a fluorinated monofunctional acrylate monomer in an amount of from about 2 to about 12 parts by weight per part by weight of the diacrylate monomer; PA1 c) a photoinitiator; and PA1 d) a viscosity modifying agent in an amount sufficient to increase the viscosity of the composition to a value in the range of from about 1000 to about 15000 cP.
Example 4 of this application shows the production, from a preferred photopolymerizable composition, of a cured polymer having a refractive index of 1.378.
Although the claddings described in this copending application are satisfactory for many purposes, there are certain applications, for example, coupling the output of a laser diode to an optical fiber, where a cladding with an even lower refractive index is desirable in order to provide an optic fiber with a higher numerical aperture and coupling efficiency. However, the present inventor has found that if one attempts to produce a very low refractive index cladding using the compositions described in this copending application by employing a highly fluorinated diacrylate monomer, a highly fluorinated monoacrylate monomer and, as the viscosity modifying agent a homopolymer of a highly fluorinated monoacrylate, unsatisfactory results are obtained. Firstly, during polymerization of a solution of the viscosity modifying agent in the monomer mixture, substantial haze develops, apparently because of phase separation between the polymer being produced and the viscosity modifying agent. Secondly, most available photoinitiators are insoluble in the mixture of highly fluorinated acrylate monomers, and even diethoxyacetophenone, which is somewhat soluble in the monomer mixture, becomes incompatible and gives very hazy mixtures in the presence of the viscosity modifying agent.
It has now been discovered that the aforementioned problems can be eliminated, or at least substantially reduced, by first reacting a photoinitiator monomer having both a photoinitiating group and an ethylenically unsaturated group with a fluorosubstituted monomer having an ethylenically unsaturated group to form a copolymer having pendant photoinitiating groups, and then using this copolymer as a combined viscosity modifying agent and photoinitiator in a photopolymerizable composition.